secrecy: use const generics, remove array impl macros

[nilslice]

This PR removes the impl_cloneable_secret_for_array and impl_debug_secret_for_array macros from the secrecy crate in favor of using const generics.

The doctest in secrecy has been updated to illustrate the usage on a [AccountNumber; 3] type as well.

I am not 100% certain that the usage is correct, but if so, I think it is also more thoroughly demonstrated now that it includes the Debug impl on a type which delegates to the trait's default DebugSecret::debug_secret method.

[nilslice]

@tony-iqlusion @Shella - just came across this old PR in a clean-up pass of my Github 😄

Should I close it or is this something you're open to?

[tony-iqlusion]

We're still potentially interested. Please don't close it.